Prospective regions of eastern Colorado are divided naturally into two main classes: the basins which have in the past, because of depths, been thought of as limited to Cretaceous possibilities; and those areas in which the Paleozoic formations present the principal, or only, chances of production. With the advent of deeper drilling techniques the phantom barrier to prospects below the Cretaceous in the basin areas tends to disappear. This barrier is also modified by the fact that excessive drilling depths, formerly predicated for the deeper parts of the basins, can be scaled down in line with sections measured along the Rocky Mountain front, and from a few scattered well logs in the east.

The areas of Cretaceous prospects are the three basins in the western part of the region. Oil and gas have long been produced from all three of these basins and the recent discovery in southwestern Nebraska has centered much attention on that and other parts of the Denver basin. Additional prospects exist within the Cretaceous, and beneath the Cretaceous, especially in large parts of the Denver basin.

The areas which have heretofore been considered the particular loci of Paleozoic prospects are the Sierra Grande uplift and the so-called Las Animas arch, plus the territory north and northeast toward the northeast corner of the state. The Apishapa uplift, connecting the Sierra Grande uplift with the Wet Mountains, and separating the Denver and Raton basins, has been included in this area of Paleozoic prospects. Parts of these areas still hold excellent prospects, especially on the flanks of the uplifts, and in areas east and north.

In the literature Sierra Grande uplift and Las Animas arch have been used as synonymous terms. The Sierra Grande uplift is a very old feature in New Mexico and southern Colorado, and the so-called Las Animas arch is a much younger feature due mainly to the subsidence of the Denver basin contemporaneous with Cretaceous sedimentation, and probably modified by later Laramide orogeny. The northwestward continuation of the Anadarko basin, in the older rocks, may be traced directly across the present surface expression of the Las Animas arch.

The geologic history and paleogeography are briefly traced to show what the writer believes to be excellent prospects in the Paleozoic formations in the deeper parts of the basins, especially the Denver basin, at what are not drillable depths.

End_of_Article - Last_Page 623------------